Source: Paul Crabtree

Root zone: Tips for healthy street trees

This drawing offers a lot of information for the maintenance of healthy street trees, which are critical to urbanism. A rule of thumb is that tree roots will not penetrate soils compacted to greater than 85 percent. Most concrete sidewalk specs call for 95 percent compaction in order to cover worst case scenarios. However, research and experience show that most sidewalk failures occur due to differential settlement; and that uniform compaction is more important in preventing differential settlement than compaction percentage is, and also that a few inches of granular material between the soil and the concrete helps alleviate differential settlement.  A basic protocol:

1. Try to not over-compact existing soils. Use smaller equipment in those areas when possible.

2. In building up soils to accept tree roots compact to 85%, and do it in layers of 8-12 inches at a time of friable soil (optimum moisture content) to ensure uniform compaction.

3. Use a few inches of pea-sized fractured rock under sidewalks.

4. Concrete sidewalk thickness should be at least 4 inches, and consist of high quality concrete that is properly cured.